污染场地六价铬迁移转化机制与数值模拟研究

随着全球工业化迅猛发展，土壤和地下水六价铬污染日益严重。基于某铁合金厂铬渣场地现场调查与采样分析，开展铬渣场地土样吸附、渗透和弥散试验，研究六价铬在粉质黏土土样中的吸附特性和迁移规律，建立考虑对流-弥散-吸附的六价铬迁移三维动力学模型，结合数值软件获取污染源位于场地上、下游时地下水中六价铬迁移分布特征，并揭示弥散度?和分配系数 对六价铬时空分布的影响。试验结果表明，粉质黏土对六价铬吸附符合Langmuir等温吸附模型，最大吸附量为466.6 mg/kg；蒸馏水和160 mg/L 六价铬溶液入渗下粉质黏土渗透系数约为6.5×10–7～6.7×10–7 cm/s，1 000 mg/L六价铬溶液的渗透系数增大至4.4×10–6 cm/s；粉质黏土水动力弥散系数D为1.4×10–4 m2/d，计算得到阻滞因子 为4.2～10。数值模拟结果表明，场地下游受到六价铬污染时，即使不考虑分子扩散作用，上游仍有被污染的风险，污染程度取决于含水层的弥散度；考虑含水层对六价铬吸附时，土体分配系数越大，六价铬污染羽分布范围越小，在预测地下水中六价铬浓度分布时应重点考虑六价铬吸附等转化过程。

Numerical simulation on the migration and transformation mechanism of hexavalent chromium in contaminated site

With the rapid development of global industrialization,the pollution of hexavalent chromium(Cr(Ⅵ))in soil and groundwater has become increasingly serious.Field investigation and laboratory tests were carried out for the soil polluted by the chromium slag of a ferroalloy plant.The adsorption,infiltration and dispersion experiments were conducted to study the adsorption characteristics and migration mechanism of Cr(Ⅵ)in silty clay.A three-dimensional kinetic mathematical model of Cr(Ⅵ)migration considering convection-dispersion-adsorption was established.The migration and distribution characteristics of Cr(Ⅵ)in groundwater with the pollution source located upstream or downstream of the contaminated site were obtained using the numerical approach.Meanwhile,the effects of dispersity(α)and distribution coefficient(K_d)on the spatial and temporal distribution of Cr(Ⅵ)were revealed.The experimental results show that the Langmuir isotherm model well fits the adsorption data of silty clay.The maximum adsorption capacity of silty clay for Cr(Ⅵ)was 466.6 mg/kg.The hydraulic conductivity of silty clay under the infiltration of distilled water and 160 mg/L Cr(Ⅵ)solution was 6.5×10^–7–6.7×10^–7 cm/s,while it increased to 4.4×10^–6 cm/s under infiltration of Cr(Ⅵ)solution with a concentration of 1000 mg/L.The hydrodynamic dispersion coefficient(D)of silty clay was 1.4×10^–4 m²/d.The value of the retardation factor(Rd)was found to be 4.2–10.The results of the numerical simulation indicated that when the downstream was contaminated by Cr(Ⅵ),there was still a risk of pollution in the upstream even if molecular diffusion was not considered.The degree of pollution depended on the dispersity of the aquifer.Considering the adsorption of Cr(Ⅵ)by the aquifer,the higher the soil distribution coefficient,the smaller was the distribution range of the Cr(Ⅵ)pollution plume.Therefore,the transformation processes such as Cr(Ⅵ)adsorption should be focused on when predicting the distribution of Cr(Ⅵ)in contaminated sites.